Acrylic pressure-sensitive adhesive (PSA) compositions that can be coated as a hot melt (100% solids) or as a high solids coating ( greater than 60% solids) and crosslinked by ultraviolet (UV) light are provided. The adhesives are based on copolymers of alkyl acrylate(s), ethylenically unsaturated carboxylic acid(s), N-vinyl lactam(s), and ether monomer(s), the latter serving as a photosynergist and containing at least one labile hydrogen atom that is easily abstracted during the UV crosslinking step. The polymers are compounded with a photoinitiator and a multifunctional acrylate prior to coating and, after being cured, yield high-performance PSAs.
Solvent-based, acrylic, pressure-sensitive adhesives (PSAs) are used for high-performance applications because of their excellent balance of peel adhesion and cohesive strength. In general, the constituent polymers of such adhesives have high molecular weights, and the adhesives are characterized by high relative viscosity. Consequently, dilution with solvent is required to make the polymers coatable at ambient temperatures. They are typically coated at a solids content of from 30 to 40% by weight. The solvent is removed (evaporated) immediately following the coating process.
The lower the solids content of the solvent-based adhesive, the more time is required to drive off the solvent after coating. Consequently, it is necessary to employ very slow line speeds during the coating process to ensure a solvent-free coating that has no defects, such as bubbles or skinning. As a result, the cost of manufacturing solvent-based acrylic products is high.
Acrylic hot-melt adhesives were developed to try to match the adhesive performance of solvent-based adhesives while lowering the cost of coating by increasing line speeds. The molecular weight of such adhesives has to be lower than traditional solvent-based acrylics to ensure that the viscosity of the adhesive (100% solids content) is kept within a processable range. This lower molecular weight results in a lower cohesive strength, as shown by low shear performance. Therefore, the adhesive needs to be crosslinked after coating. Unfortunately, excessive crosslinking results in low adhesive peel and tack performance. In addition, hot-melt adhesives have often contained high residual monomers, a characteristic that is unacceptable for medical applications where the adhesives are to be applied to the skin.
U.S. Pat. No. 5,686,504 describes pigmented, crosslinked, pressure-sensitive adhesives that exhibit high cohesive strength. The adhesives are UV-crosslinked through tertiary amine groups (incorporated into the backbone of the adhesive copolymer) and hydrogen-abstracting photoinitiators, which act synergistically and facilitate free radical formation and crosslinking upon exposure to UV light. The adhesives described in the ""504 patent are a significant improvement over previous UV light-cured PSAs. However, the use of tertiary amine monomers in the adhesive copolymer precludes the use of acid-containing monomers, like acrylic acid, which could otherwise be used advantageously to make an adhesive copolymer having desirable properties. Such acid monomers must be avoided to ensure that the amine groups remain unprotonated and available for crosslinking by UV light.
A continuing need exists not only for high performance PSAs, but also for compositions that can be coated and crosslinked to form such PSAs, particularly compositions that can be coated both as a hot melt and as a high solids content coating.
In accordance with a first aspect of the invention, a coatable, acrylic composition comprises, on a percent-by-weight basis, (a) about 85-99.7% of at least one acrylic copolymer; (b) about 0.1-5% of at least one photoinitiator; and (c) about 0.2-10% of at least one multifunctional (meth)acrylate. The acrylic copolymer(s) are formed from a mixture of monomers comprising, on a percent-by-weight basis, based on the total weight of monomers, (i) about 67-93% alkyl acrylate(s), each having an alkyl group with 4-8 carbon atoms; (ii) about 5-20% vinyl lactam(s); (iii) about 1-5% ether monomer(s); and (iv) about 1-8% ethylenically unsaturated carboxylic acid(s); wherein the sum of all monomers is 100%. The composition can be coated as a hot melt (100% solids) or as a high solids content solution ( greater than 60% solids), at high line speeds, using conventional coating technology. The composition is readily crosslinked by exposure to ultraviolet (UV) light to yield a high-performance acrylic PSA.
In a second aspect of the invention, a crosslinked PSA tape, label, or similar construction comprises a crosslinked acrylic PSA in contact with at least one substrate. The crosslinked acrylic PSA is obtained by curing (crosslinking) an acrylic composition as described above.
The new compositions can replace existing solvent-based adhesives used in industrial applications that are currently coated at  less than 35 feet per minute (fpm), allowing reduced manufacturing costs and increased solvent coating capacity. The capability of being coated as hot melts (100% solids) makes them attractive for use in the manufacture of very thick coatings ( greater than 5 mil) for high performance industrial tapes.